This application relates to an apparatus and method for using attenuated total reflectance spectroscopy to analyze surface defects of polymer surfaces.
Internal Reflectance or ATR (Attenuated Total Reflectance) is a widely used Infrared technique for studying materials that provide good optical contact with the beam directing IRE (Internal Reflectance element). Attenuated total reflection utilizes the phenomenon that, under the condition of near total reflection at the interface between a highly refractive transparent medium and the electromagnetic radiation absorbing medium under investigation, such radiation incident on the reflecting interface actually enters the absorbing medium. The reflected radiation is attenuated by this minute penetration and provides a spectrum having absorption bands characteristic of the absorbing medium under investigation. Thus the interaction of the light beam (infrared) with the sample surface at the interface of the sample and the element provides spectral information about the sample. The depth of penetration of the IR radiation into the sample surface can be controlled by varying the angle of incidence of the beam striking the sample-IRE interface. The depth of penetration is also dependent upon the wavelength of radiation and refractive indices of the element (IRE) and the sample, and is defined according to equation 1. ##EQU1## where D.sub.p =depth of penetration
.lambda..sub.1 =wavelength of radiation PA1 n.sub.2 =refractive index of sample PA1 n.sub.1 =refractive index of the element PA1 .alpha.=angle of incidence
There has been a phenomenal growth in IR microspectroscopy as applied to material sciences in past decade or more. There is a great potential for future growth. Early limitations of micro-transmission and micro-reflectance appear to have been overcome by the introduction of internal reflectance IR micro-spectroscopy. Now samples can be characterized by acquiring spectral data representative of only a few microns from the surface of trace areas. A set of new IR directing elements have been developed which not only allow sampling of the trace surface selectively but at the same time allow the user to control the depth (depth profiling) to which the IR radiation samples the material. This is accomplished by changing selected parameters of equation 1. Depth profiling has become a valuable tool in studying surface defects of certain materials. Specifically, the surface defects of bloomed materials, gel particles, smudges and different types and shapes of domains characteristic of synthetic polymers can now be better analyzed. Opaque samples such as small particles, single fibers, biological samples, tissue samples and forensic samples can be easily characterized which are otherwise difficult to analyze by microtransmission or micro-reflectance IR spectroscopy.